Debit cards are currently in use for the operation of vending machines and the like, frequently in substitution for coin operated machines. Cards bearing a magnetic stripe, previously encoded with a starting credit balance, are inserted in such machines and successively debited as each transaction is made. The starting credit balance is read from the card as it is inserted into the reader-writer, whose microprocessor interprets and verifies the reading. The balance so read is displayed visually. The microprocessor of the reader-writer then successively debits the displayed balance as purchases or other debiting transactions are made. When desired transactions have been completed, the card is rewritten, as it exits from the reader-writer, with the new credit balance from the microprocessor memory, and the memory is canceled.
Such automated systems seem inherently to challenge persons to fraudulent trickery. Manually operated card reader-writers, in which the user inserts the card only partially, leaving a part of it exposed for withdrawal, are particularly subject to such fraudulent use. One wholly obvious manner of fraudulent use which might be feasible, would be to utilize the card to effect a series of transactions which debit substantially its entire credit amount, then to disconnect the power to the machine, then to withdraw the card with the encoding of its original credit balance undiminished. There are several less obvious procedures by which such fraudulent use has sometimes been effected; it would be inappropriate in this specification to teach such procedures.
Such problems are largely avoided in motor driven card readers, which draw the card entirely into the machine and beyond the reach of the user. After the credit amount of the card has been read into the memory of the machine, the motor subjects the card to a reversing movement on which the magnetic head erases the credit amount, all before any debit transactions are enabled.